We have been studying the mechanism of F-incompatibility, a function of the E. coli F factor related to the control of its replication. Incompatibility exists between an integrated F factor and an autonomous F factor (Hfr:F' incompatibility) and between 2 different autonomous F factors (F':F' incompatibility). We have genetic evidence that the underlying mechanisms for these 2 kinds of incompatibility are different. There are 3 ways in which an F factor can overcome the incompatibility barrier: mutation, integration into the chromosome and fusion with another F factor to form a single F factor. Our future studies are aimed at genetic analysis of the mechanisms of both types of incompatibility. For Hfr:F' incompatibility, our approach is mainly genetic: isolation of Inc minus mutants, study of their properties and mapping. For F':F' incompatibility, our approach is a combination of genetic and physical methods, including electronmicroscopy, to elucidate the process of fused episome formation. In addition, we shall continue our studies, which we have developed in conjunction with this project, on the isolation and characterization of phi 80 trans-ducing phages which have incorporated various regions of the E. coli chromosome into their genome. This is a general method for the amplification of specific E. coli genes.